1. Field of the Invention
This invention relates to an alternator for a vehicle such as a passenger automotive vehicle or a truck.
2. Description of the Related Art
To reduce the aerodynamic resistance in a traveling condition, a vehicle body is often formed into a slanted nose shape. Securing a sufficient residential space for a passenger compartment is earnestly demanded. To satisfy these requirements, engine rooms of automotive vehicles have recently been becoming so narrow and crowded that only a limited space is available for installing an alternator. In addition, the temperature of a region around the alternator is often high. Meanwhile, to improve fuel economy, the rotational engine speed tends to be reduced during an idling condition. The rotational speed of the alternator decreases in accordance with the reduction of the rotational engine speed. On the other hand, there is a need for supplying increasing electric loads such as safety control devices, etc. Thus, a high power generating ability of the alternator is strongly required. In other words, a compact high-power alternator for a vehicle is required. Especially, it is desirable that the power generating performance of an alternator is good even when the alternator is operated at a low rotational speed.
Furthermore, it is highly desirable that noises leaking from vehicles should be reduced. Providing a quiet passenger compartment leads to increased product appeal. To this end, the engine noises have recently been lowered. On the other hand, alternators for vehicles which operate at high rotational speeds tend to generate fan noises which may annoy passengers and, wind blocking sounds, and magnetic sounds.
In a general alternator for a vehicle, the greatest heating source is the stator. To attain a high power output and a high efficiency of an alternator, it is conceivable to reduce the resistance of a winding on the stator and thereby to reduce heat loss. Specifically, an electric conductor having a great cross-sectional area is used for the winding. In addition, it is conceivable to increase the ratio of a conductor-occupied area in each slot in the stator to an unoccupied area. This ratio is hereinafter referred to as the space factor.
In a prior-art alternator for a vehicle, a stator has inner circumferential surfaces opposing a rotor and is formed with teeth. Tips of the teeth have magnetic-flux collecting projections extending in circumferential directions. Therefore, open ends of slots in an iron core of the stator are smaller in width than inner portions of the slots. The thickness of an electric conductor for a winding is limited by the width of the open end of each slot. To increase the power output of the prior-art alternator during operation at a low rotational speed, a great length of the magnetic-flux collecting projections is necessary. As the length of the magnetic-flux collecting projections is increased, the open ends of slots are narrower so that a thinner electric conductor is required for the winding. The thinner electric conductor causes an increased resistance of the winding.
Japanese published unexamined patent application 63-194543 discloses that an electric conductor for a winding has portions to be placed in slots, and these portions are previously made into a shape having an approximately rectangular cross-section as shown in FIG. 10. The portions of the electric conductor are placed into the slots as shown in FIG. 11(A). Then, edges of a stator around the slots are plastically deformed into magnetic-flux collecting projections as shown in FIG. 11(B).
In the prior-art structure of Japanese application 63-194543, the plastic deformation needed to make the magnetic-flux collecting projections results in deteriorated magnetic characteristics. As a result, the magnetic-flux collecting effect is reduced. Thus, it is difficult to attain a desired alternator power output. Especially, it is difficult to provide an increased alternator power output during operation at a low rotational speed.
In the prior-art structure of Japanese application 63-194543, the plastic deformation causes a strain in the stator, decreasing the circularity of the inner circumferential surfaces of the stator and hence forming an uneven air gap between the stator and a rotor.
The uneven air gap increases magnetic noise during operation of the prior-art structure.